The present invention relates to the construction of a multi-region reactor core pebble bed high temperature gas reactor. In a gas reactor of this type, the reactor core is formed by using spherical fuel produced by dispersing coated fuel particles in a graphite matrix and hardening the particles by burning in spheres each having a diameter of approximately 6 cm. This type of reactor has features that permit refueling during operation and is also characterized by high temperature heat supply, high inherent safety, high degree of burn-up of fuel and so forth, and the development thereof is being advanced in various countries.
At present, the development of the pebble bed high temperature gas reactor is tending toward to types, a monolithic type and a modular type.
The monolithic type pebble bed reactor is constructed by assembling a pebble bed reactor core, a reactor core portion of a graphite reflector, and principal equipment such as a refueling device, a control rod, a gas circulator and a steam generator in a prestressed concrete pressure vessel having a single cavity. In West Germany, a prototype reactor (THTR) on a scale of electric power at 300 MW is already in test operation, and a project of a plant (HTR-500) on a scale of 500 MW is also being promoted.
On the other hand, in the modular type pebble bed reactor, design items of the reactor core such as the core size, thermal power density and heavy metal charging quantity per fuel sphere are selected so as to further increase the inherent safety. The modular type pebble bed reactor has a module construction in which a pebble bed reactor core and primary principal apparatus are housed in a steel pressure vessel to thereby unite them in a unit after limiting the diameter of the reactor core to such a diameter (approximately 3 mm) that makes it possible to shut down at low temperature only by simply inserting a control element into the reflector region (an interior of the reflector itself). The output, however, is comparatively small, and the electric power of a single reactor is at approximately 80 MW.
However, there are problems in the above-mentioned monolithic and modular type pebble bed reactors.
In the monolithic type pebble bed reactor, the dimension of the reactor core diameter becomes large, and nuclear control for a large-sized reactor having a large output scale is insufficient in a cooled state using only control elements (control rod, boron sphere) inserted into the reflector region. Furthermore, a reactor core control rod, which is forcibly inserted directly into the reactor core, is required in addition to the control elements inserted into the reflector region by gravity drop only. Moreover, cases have been reported that fuel spheres are broken if the control rod is inserted forcibly into the core because the pebble reactor core is full of fuel spheres accumulated inside the reactor core, which causes a serious drawback.
Another problem is that a supporting structure for supporting the load of the top reflector is troublesome in a reactor of large power scale because the top reflector, manufactured with graphite blocks, becomes increasingly large as the diameter of the reactor core becomes larger. This, in turn, requires a troublesome structure for supporting the top reflector by suspending it from the ceiling of a concrete pressure vessel.
In the case of building the reactor in sites where an earthquake may be probable, it is more difficult to attain the structure for supporting the top reflector in terms of design.
In a modular type pebble bed reactor, the reactor can be stopped at low temperatures without requiring the insertion of the control rod into the reactor interior, thereby obviating the foregoing problems peculiar to the monolithic type reactor. On the other hand, it is required to have the modular type pebble bed reactor correspond as a multi-module plant where a plurality of sets of modular type reactors are arranged in order to form a plant of large output scale since the output of a single set is as low as approximately 80 MW. However, with numerous modular type reactors in the same site, the equipment assumes a large configuration for the required output scale. This in turn causes economic disadvantages as well as problems with site availability.
The side reflector constituting the core frame of the pebble bed reactor is constructed typically of graphite blocks, and hence there is a possibility of damage to the reflector blocks due to thermal deformations when effecting excessive irradiations of high-speed neutrons during the operation of the reactor. If damaged, however, the prior art devices do not provide any particular means for replacing or repairing the damaged reflector blocks by remote control from outside of the reactor. This situation is likely to be a critical problem in terms of safety and supervision, which is to be solved from now on into the future.
Accordingly, it is a primary object of the present invention to eliminate the foregoing problems pertaining to the nuclear controllability in the monolithic type reactor, by providing a novel pebble bed high temperature gas reactor capable of producing outputs of several hundreds of MW with a small-sized and compact configuration.